For flashing a bootloader yes, you need a UART (there may be options like RS232<->UART converter) and the easiest way is tu buy a USB<->TTL adapter. Cheap like hell. Can do your own as well (e.g. roboforum.ru/forum4/topic10592-15.html#p232256 - sorry, it’s in Russian). If you use a scope you need other convenient tools, so it’s a good investment. You can also consider more complicated tools like BusPirate. Does an awful lot of things but it will take time to learn how to handle it.
Also, there was a piece of software that did *.hex to *.bin convertation. Will try to find it for you - can’t remember what’s the name of it…